Remove GTK dependencies

[m3h]

Related to #4

Currently, there are some GTK dependencies in the main loadshedding.py script.

loadshedding/loadshedding.py

Lines 192 to 195 in 7dd5761

	from gi.repository import Gtk
	import notify2
	main()

Now it's been a while since I took a look at this code, but if I recall correctly, the GTK dependency existed purely to be able to cancel the notifications, and cancel the shutdown/script execution.

I just had a look, and the notify2 package is deprecated (https://web.archive.org/web/20210714093727/https://notify2.readthedocs.io/en/latest/), so that dependency also needs to be removed.

But I really liked the ability to cancel the action as a user on GUI systems, so I want to keep it in.

[m3h]

So I haven't found a platform-independant notification library that allows for actions.
I might go the kind of clunky, but quite workable solution of using PySimpleGUI (https://pysimplegui.readthedocs.io) and loading up a GUI Window with some buttons and a timeout instead.

[m3h]

Some further research seems to indicate that tk/tcl is included with standard Python installations.
That would be the preferred option, to keep the dependencies lean.